MAGNETOMETERS

1. MAGNETOMETERS Kedrick Black ECE 5320 Mechatronics Assignment #1 Kedrick Black

2. Definition of attitude control Description of a magnetometer How the magnetometer works in general Types of magnetometers Real time monitoring of the earths magnetic field How the magnetometer works in space First magnetometer used in space The ideal magnetometer Design tradeoffs for magnetometers Example placement on spacecraft Calibration Testing Fluxgate magnetometer Honeywell 3-axis magnetometer Limitations Places to buy/Cost OUTLINE Kedrick Black

3. www.aa.washington.edu http://www.izmiran.rssi.ru/magnetism/Petrov/dolgino_rep.html http://www.georentals.co.uk/TR120-new.PDF http://www.sp.ph.ic.ac.uk/~balogh/spinstr.htm http://www.magnetometer.com/attcntrl.htm#rad Reference List Kedrick Black

4. www.goodrich.com http://www.lund.irf.se/HeliosHome/geomag_magnetometers.html http://www.sp.ph.ic.ac.uk/~balogh/spinstr.htm http://server4.oersted.dtu.dk/research/CSC/publica/Papers/Magcalibchamp_PB.PDF server4.oersted.dtu.dk/research/ CSC/publica/Papers/ESASP_Acalib.PDF To Explore Further (Web Pointers) Kedrick Black

5. Definition: Attitude is the 3-D rotation between the coordinate system of a known object and that of a sensed portion of its surface. Combinations of the support function of a known object with curvature measurements from a visible surface transform attitude determination into optimization problems that can be solved using standard numerical methods. Previous work using the extended Gaussian image (EGI) defined for convex polyhedra is extended to the domain of smooth, strictly convex objects where the EGI becomes equivalent to the second curvature function. Three-dimensional shape matching using the first curvature function is new. Emphasis is placed on theoretical foundations, algorithm development, and experimental proof-of-concept using real objects and surface data obtained from an existing photometric stereo system • Spacecraft attitude systems usually consist of two components: • vehicle orientation sensors: e.g. magnetometers, horizon sensors, sun sensors, gyroscopes, and star trackers; and • - active attitude control actuators: e.g. control momentum gyros, reaction wheels, offset thrusters, and magnetic torque rods. Kedrick Black

6. Instrument for measuring the strength and sometimes the direction of magnetic fields, including those on or near the Earth and in space. Magnetometers are also used to calibrate electromagnets and permanent magnets and to determine the magnetization of materials. Magnetometer Kedrick Black

7. Magnetometer measures the magnetic field it is applied to. The magnetometer outputs three magnitudes: X, Y and Z. From these three values you can construct the magnetic field vector (magnitude and direction): B= [X, Y, Z]. How Magnetometers Work Kedrick Black

8. Proton Magnetometers: A coil surrounds a working sample which is usually a hydrogen containing liquid. The liquid contains protons which have a known gyromagnetic ratio of 2.67515341x10^8 radians/sec/Tesla. The surrounding magnetic field can be calculated within .004nT. Preferred application: Where the absolute accuracy of the magnetometer is important and the low cycling rate will not be a problem. Overhauser Magnetometers: The Overhauser magnetometer can be likened to a laser or maser. A high frequency RF signal provides the pumping energy to keep the protons constantly processing. A tuned circuit built around the sensing coil functions much like the cavity in a maser. Because of the physical nature of the proton this error in field strength measurement will be less than approximately 25nT. Preferred application: Where the faster sampling rate is important, the absolute accuracy of the free precession magnetometer is not needed. Optically Pumped Magnetometers: All of the optically pumped magnetometers use the properties of the electron to make their measurements. Light at a particular wave length is radiated through a gas cell containing the working gas of the magnetometer and from there onto a photocell. A small coil, called the H1 coil, applies a small RF field to this gas cell. Preferred application: Where one time adjustment and calibration is needed. Fluxgate Magnetometer: consists of three sensors, each of which, together with the sensor-specific electronics, provides a voltage proportional to the value of the component of the magnetic field along its axis. The magnetic field vector is measured by an orthogonal arrangement of three sensors. Preferred application: Space where low power and mass are needed. Vector Helium Magnetometer: based on the response of an optically pumped metastable He population (contained in a glass cell) in the presence of external magnetic fields. Such fields are detected by measuring the transmission of infrared light through the He cell; the transmission coefficient is proportional to the component of the external magnetic field along the optical axis of the He cell. Preferred application: Space where mass and power are not an issue and greater sensitivity and low noise are needed. Types of Magnetometers Kedrick Black

9. Real Time Monitoring of The Earths Magnetic Field Credit: http://geomag.usgs.gov/ Kedrick Black

10. How the Magnetometer Works In Space At a given point in space and time we can calculate the magnetic field from a model (IGRF2000). By comparing the field direction the magnetometer is telling us with the model we can determine how the magnetometer must be aligned to the field direction to give those X, Y and Z results. Kedrick Black

11. First Magnetometer for Space research • Tested and Flown of Sputnik • First successful data collected in 1958 Credit: http://www.izmiran.rssi.ru/magnetism/Petrov/dolgino_rep.html Kedrick Black

12. The ideal Magnetometer for space • Small • Cheap • Light • Low power • Insensitive to noise • High Resolution • Large temperature range • Sustainable to high shocks Kedrick Black

13. DESIGN TRADEOFFS FOR SPACECRAFT ATTITUDE CONTROL SENSORS Kedrick Black

14. Example Placement on a satellite Credit: Department of Aeronautics & Astronautics University of Washington Kedrick Black

15. Calibration of the Magnetometer Bcal t = I +S Bmeas t) +b = A(t)Bmodel (1) where, I = Identity matrix S = Scaling and orthogonality matrix b = Bias (offset) vector A = Attitude rotation matrix Magnetometer calibration improvement Credit: http://staff.ee.sun.ac.za/whsteyn/Papers/Magsat.pdf Kedrick Black

16. Testing • An electric circuit can be connected to the magnetometer which is connected to a microprocessor which is connected a PC. • The values the magnetometer are measured and it is rotated and the process is repeated Kedrick Black

17. University of Washington Testing • The magnetometer was placed on a board in a way so that the magnetometer’s x-value was pointing straight up and we rotated the magnetometer in the Y-Z plane. Credit: Department of Aeronautics & Astronautics University of Washington Kedrick Black

18. U of W Experiment Data Ideal Data Actual Data Credit: Department of Aeronautics & Astronautics University of Washington Kedrick Black

19. A Simple Fluxgate magnetometer Credit: http://www.tuc.nrao.edu/~demerson/magnet/magnet.html Kedrick Black

20. The TFM100S Fluxgate Magnetometer Credit: http://www.magnetometer.com/attcntrl.htm#rad Kedrick Black

21. Honeywell 3- Axis MagnetometerHMC2003 Credit: www.honeywell.com Kedrick Black

22. High Power Consumption Low Sensitivity Large Mass Susceptible to other magnetic disturbances Fluxgate is only type easily available for space use Limitations Kedrick Black

23. Honeywell Goodrich Structure Probe Inc. LessEMF.com Cost aprox. $5000.00 Places to buy/Cost Kedrick Black